Relay
An electromechanical component that switches a much larger load with a small coil current. The coil typically draws 120–200 mA at 12 V and 60–100 mA at 24 V, while the contact side can carry 10–80 A. It provides both isolation and current gain between the control circuit and the power circuit.
When voltage is applied to the coil, the resulting magnetic field pulls an armature and closes the contacts. The control side and the power side are physically separate; that separation is what lets a low-current microcontroller output drive a motor or valve load of tens of amperes.
- Contact form: NO (normally open), NC (normally closed) and changeover are the three most common types. Direction-reversing circuits use two relays with changeover contacts.
- Contact current: the rated value on the label is for a resistive load. With inductive or inrush-current loads such as motors, lamps and coils, it is safe to stay within a continuous current of typically half the label value.
- Coil voltage: it must match the system voltage; a 12 V coil burns out quickly on a 24 V line.
On the relay-output models of AXI remote controls and drivers, you have to check the coil current and the contact load separately when driving an external load. If you are switching an inductive load, adding a reverse diode across the coil and a snubber or varistor across the contact extends contact life noticeably.
Context in which this term is used: Installation, redundancy and field continuity